144 



OBOLUS LENS, n. sp. PI. X., Figs, a to f. 



(TMlens Shell rather thin and fragile, lenticular, nearly orbicular, with the 



described. beak of the ventral slightly projecting. 



Ventral valve with a depressed beak, slightly sloping sides, and usually 

 a depressed line, running along the mesian area. Interior, with a car- 

 dinal area about one-eight of the length of the valve. Within is a 

 visceral callus, extending a little beyond the middle of the valve, and 

 about twice as long as wide ; at the front is the scar of the central 

 muscles ; on the callus, two-thirds from the beak, is a shallow lance oval 

 depression. In front of the callus, but not always present, is a mesian 

 ridge, bordered by shallow grooves. Scars of the lateral muscles are 

 visible at the sides of the valve near the hinge line, outside of a long 

 vascular ridge. 



The dorsal valve shows wide, flattened areas at the posterior lateral 

 slopes, and a narrow, widening, depressed band along the median line ; or 

 the centre is scarcely depressed, and there are two flattened ridges on 

 the valve diverging from the uinbo, and about as far apart at the front 

 as one quarter of the length of the valve. Elsewhere the slopes of the 

 valve are regular. Interior. This is imperfectly known. The visceral 

 callus extends beyond the middle of the valve, as is shown by the radi- 

 ating septa that cross the visceral cavity ; two of these ridges extend 

 nearly to the front of the valve. 



Sculpture. This consists of fine concentric ridges that sometimes run 

 together ; there are about eight of these ridges in the space of a millimetre, 

 and they are nearly regularly spaced except near the umbo ; there are 

 about twice as many of these ridges to the millimetre as in 0. Breton- 

 ensis. 



Size. The ventral valve is about 14 mm. long, 12 mm. wide, and 1J 

 mm. deep. The dorsal is about 1 mm. shorter than the ventral. 



Horizon and locality. In assises E. 3 b and E. 3 e, (especially the 

 latter) of the Upper Etcheminian, Dugald brook, Escasonie, N.S. 



This species is provisionally placed with the sub-genus Palseobolus. 

 though the vascular trunks are not so close together as in that species 

 (neither are they so wide apart as in the Obolus of Eichwald (Michwitz). 

 Though proportionately a narrower shell than the proceeding, 'this species 

 is evidently congeneric with it, and appears to replace it in the more 

 sandy beds of this zone. It can be distinguished from 0. Bretonensis 

 by the closeness of the concentric ridges on the surface of the valves. 

 The crests of these ridges are not beaded as in Obolus monilifera, Linrs. 

 of the Swedish Cambrian. 



The following table shows the size and main features of a number of 

 valves. 



